Question:I know people tend to use 5v*sensor reading/1024 to get the feed back voltage. And i know it is not accurate, after googling i have found this to get the real voltage reading, codes are here :

Probably better to state what your accuracy desires are, what range voltages you wish to measure, and what minimum step size resolution you need. Then possible references can be offered. The built in arduino ADC is very useful and easy to use, but it should not be confused with instrumentation quality ADC. There are lots of I2C and SPI driven ADC chips available these days if your specifications require the performance they bring to the table.